As one of methods for a synthesizing liquid fuel from a natural gas, a GTL (Gas To Liquids: a liquid fuel synthesis) technique of reforming a natural gas to synthesize a synthesis gas containing a carbon monoxide gas (CO) and a hydrogen gas (H2) as main components, synthesizing hydrocarbon compounds (FT synthesis hydrocarbons) using this synthesis gas as a feedstock gas by a Fischer-Tropsch synthesis reaction (hereinafter referred to as “FT synthesis reaction”), and further hydrogenating and fractionally distilling the hydrocarbon compounds to produce liquid fuel products, such as a naphtha (raw gasoline), a kerosene, a gas oil, and a wax, has recently been developed.
Since the liquid fuel products using the FT synthesis hydrocarbons as a feedstock have a high paraffin content, and hardly include sulfur components, for example, as shown in Patent Document 1, the liquid fuel products attract attention as environment-friendly fuels.
Meanwhile, in an FT synthesis reactor which performs the FT synthesis reaction, heavy FT synthesis hydrocarbons (heavy FT hydrocarbons) with a comparatively high carbon number flow out as a liquid from a lower part of the FT synthesis reactor. In addition, light FT synthesis hydrocarbons with a comparatively low carbon number are generated involuntarily. The light FT synthesis hydrocarbons are discharged as a gaseous by-products along with an unreacted feedstock gas, from an upper part of the FT synthesis reactor.
The gaseous by-products includes commercially available hydrocarbon compounds of which a number of carbon atoms is 3 or more (hereinafter referred to as “light hydrocarbon compounds”), along with carbon dioxide, a steam, unreacted feedstock gas (carbon monoxide gas and hydrogen gas), and hydrocarbon compounds of which a number of carbon atoms is 2 or less. Thus, if the content of the light hydrocarbon compounds increases in the gaseous by-product, the production efficiency of the liquid fuel products will decrease.
Conventionally, the gaseous by-products are cooled down to liquefy the light hydrocarbon compounds, and then the light hydrocarbon compounds are separated from the other gas components by a gas-liquid separator.
Moreover, a method for recovering light hydrocarbon compounds contained in a FT hydrocarbon-enriched emission gas by absorbing in an FT condensate while cooling is disclosed in the following Patent Document 2.